1. Field of the Invention
The present invention relates to a backlight module, particularly to a backlight module which comprises a heating device and is capable of working normally in a condition of low temperature, and relates to the technical filed of liquid crystals.
2. Related Art
Liquid crystal (LC) is a state of matter between liquids and crystals. The principle of displaying of the liquid crystal is based on the fact that molecular arrangement of the liquid crystal can be altered by applying different voltages to the liquid crystal, and therefore the quantity of light passing through the liquid crystal is controlled in order to display various kinds of images. However, the liquid crystal does not radiate light itself. Instead, it can only control passing of light, and therefore all of liquid-crystal panels need backlight sources for illumination.
Currently, most of the liquid-crystal panels are equipped with a Cold Cathode Fluorescent Lamp (CCFL) as their backlight source. The CCFL is regarded as an ideal backlight source for the liquid-crystal panels because it has advantages of having a tiny, slim lamp tube, simple structure, low surface temperature on the lamp tube, easily processing and forming, good color presentation, uniform light emission, etc.
The CCFL is a closed gas discharge tube. Two ends of the lamp tube of the CCFL are cold cathodes made of metals like nickel, tantalum, zirconium, and so on. Such cold cathodes are electrodes that are capable of emitting electrons directly without being heated. The lamp tube of the CCFL is mainly filled with inert gas, argon, and a small amount of neon and krypton both used as a catalyst for electrical discharge. Besides, a small amount of mercury vapor is also filled in the lamp tube. In the condition of low temperature, because mercury in the lamp tube of the CCFL volatilizes slowly, pressure of the mercury vapor is not high enough and mercury ions in the lamp tube is quite limited in number so as to affect initiation and light emission of the CCFL. Accordingly, initiation voltage and light-emission brightness of the CCFL relate to temperature of the environment where the CCFL is located.
Nowadays, two main reliable methods to improve the low temperature property of the lamp tube of the CCFL are as follows. The first method is preheating via an external heating source. For instance, in a well-known method of heating a liquid crystal display (LCD) module, a layer of a semiconductor film, indium tin oxide (ITO) film, is deposited on a high-intensity special glass base by means of vacuum evaporation or magnetron sputtering, and the layer of the deposited film becomes crystal-clear transparent and has certain electrical conductivity after professional processing. A heater inside the LCD module is therefore made by utilizing its electrical conductivity. However, the aforesaid method has drawbacks of complicated processing, difficult assembly and having a low yield rate. The second method is applying over-power voltages to the LCD module when the LCD module initiates in low temperature so as to raise temperature of the lamp tube of the CCFL rapidly and facilitate mercury being volatilized to mercury vapor fast in order to realize the fast initiation purpose of the lamp tube of the CCFL. However, the aforesaid second method might affect the life of the lamp tube of the CCFL and therefore is not really an ideal solution.